In recent years, rapid technical development in the field of electronics has encouraged downsizing and weight-reducing of electronic devices. As a result, there has been progress in making electronic devices become portable and cordless, and accordingly, there has also been an earnest desire for rechargeable power sources driving the devices to have reduced size, reduced weight, and higher energy density. In order to fulfill such needs, there have been developed lithium secondary batteries with high energy density. Lately, super thin and light secondary batteries having a container member made of aluminum laminate film have also been developed and commercialized.
In such secondary batteries with a container member of aluminum laminate, battery thickness changes due to expansion and contraction of electrode volume that accompany charge and discharge. This causes deformation of the battery or broadening of space between electrodes, which in turn increase resistance of the battery, resulting in a problem of lowering performance of the battery.
There has been developed a battery in which lithium titanate is used as a material component for negative electrode active material, which can avoid such a problem. Lithium titanate has little volume change accompanying charge and discharge, and as a result, has an extremely small change in battery thickness. Use of this compound as a negative electrode active material has solved the problem described above.